The present invention relates to a Pulse Width Modulation (PWM) control system for a DC motor. More particularly, the present invention relates to a spindle motor PWM control system that maintains a constant rotating speed by controlling the pulse duration based on a phase difference between a rotational reference signal and real or measured rotational signal.
In memory reading apparatus using a rotatable recording medium such as a magnetic disk apparatus and optical disk apparatus, the disk must rotate at a constant speed. A brushless type DC motor can be used as a spindle motor for this purpose. PWM control can be employed to control the on period of stator coil power pulses based on a phase difference between a rotational reference and a real rotation signal. This approach provides power control of the stator coil based on rotation of rotor and accurately maintains the motor rotation at a constant rate. One example of such PWM control is disclosed in Japanese Unexamined Patent Publication (Kokai) No. 62-149069 issued on Jul. 3, 1987.
FIG. 1 is a block diagram of a PWM control system. FIG. 2 is a timing diagram for the FIG. 1 system. FIG. 3 is a timing diagram that illustrates a relationship between motor torque coil phase and a PWM control signal in the FIG. 1 system.
In FIG. 1, a spindle motor 2 rotates a disk or recording medium 1 (e.g., a magnetic disk or optical disk) at a constant speed such as 3600 rpm. The spindle motor 2 can comprise a brushless type DC motor. Control of the spindle motor 2 is realized by controlling the power supplied to the stator coil via, e.g., a transistor switch and basing this control on the stator coil phase which is shown in waveform (b) of FIG. 2.
A reference clock generating circuit 4 (FIG. 1) generates a reference clock such as waveform (a) shown in FIG. 2. The reference clock determines the period of a single revolution of the disk 1. The real rotation detecting circuit 5 detects, via a disk read head, an index signal that is generated once for each complete rotation of the disk 1. A phase comparison circuit 6 detects a phase difference between the reference clock and the index signal. In the example waveforms shown in FIG. 2, the index signal follows the reference clock. In this case, acceleration control must be performed for the spindle motor 2.
A pulse width deciding circuit 7 controls the width of the PWM control signal as shown in waveform (e) of FIG. 2. This control is based on the detected phase difference (waveform (d) shown in FIG. 2) provided by the phase comparison circuit 6. In other words the pulse width deciding circuit varies the time Ton based on the detected phase difference. As seen in the FIG. 2 waveforms the time Ton is short compared to the time for a single turn of the disk 1. When the motor speed must be increased, the pulse duration Ton must be increased and when the motor speed must be decreased, the pulse duration must be reduced. As shown in FIG. 1, the PWM control signal is supplied to a motor driving circuit 8. The motor drive circuit 8 drives the motor 2 based on the PWM control signal; and as a result, generates a motor current is as shown in waveform (f) of FIG. 2.
However, with the above spindle motor PWM control system, the coil phase and PWM signal are asynchronous as shown in waveforms (b) and (e) of FIG. 2. Because these signals are asynchronous, the motor torque is different depending on the coil phase, even when the PWM control signal having a constant duration. Thus, motor control accuracy is lowered.
Referring to FIG. 3, waveform (b) represents motor torque. The peak torque occurs at the center of the coil phase represented by waveform (a). The minimum torque occurs at the selecting region or change of the coil phase. If the center of PWM control signal 1 occurs at the time of peak torque as shown in FIG. 3, then the motor 12 is effectively accelerated. However, if the center of PWM control signal 2 occurs at the time of minimum torque as shown in FIG. 3, then the motor is effectively decelerated. As a result, motor rotation (e.g., acceleration) is different depending on the rotational position, even during a single rotation with a PWM control signal having a constant pulse width. This causes steady phase error to become large; and thus, the accuracy of motor rotation control is lowered. The above PWM spindle motor control system is used in five inch disk drives manufactured by Fujitsu Limited in Japan and sold in the U.S.A. as model M-2663E.